I can't say it's a general trend, but my food shots sell better at IS, my illustrations do better at DT and FT (they're not at IS), my fireworks sell well at IS and not too much behind at BigStock and StockXpert. My giftboxes uploaded for Christmas sold much better at IS than anywhere else.

IS is also more open to flower shots (which DT does not accept.. period) and they seem to sell. Fotolia is a good overall seller. In the end, you can look through the top 100 images on each site to get a better idea, or browse their categories.

IS is also more open to flower shots (which DT does not accept.. period)

Now that you've mentioned, yes, I only have oe flower shot accepted in DT (many I haven't submitted because they are below 3MPix). And this only one, approved last week, got one sale (they rejected many similar ones because they have "too many").